A flush bolt is used sometimes when fastening a member. In a case of using the flush bolt, a hole corresponding to the flush bolt (hereinafter, referred to as a flush bolt hole) is provided in the member to be fastened. The flush bolt is inserted into the flush bolt hole, whereby the member is fastened. FIG. 1 is a cross-sectional view showing an example of a flush bolt hole 6. In FIG. 1, the flush bolt hole 6 provided in a target member 3 is shown. The target member 3 has a first member 1 and a second member 2, and the first member 1 and the second member 2 are laminated. The first member 1 is a composite material. The second member 2 is a composite material or a metal material (Ti, Al, and the like). The flush bolt hole 6 extends toward a back surface 5 of the target member 3 from a principal surface 4 of the target member and has a countersunk head portion 7 and a constant portion 8. The countersunk head portion 7 is connected to an opening provided in the principal surface 4 and configured such that a diameter decreases as it becomes deeper. The constant portion 8 connects a bottom portion of the countersunk head portion 7 and an opening provided in a back surface 5. The diameter of the constant portion is constant in a depth direction. A flush bolt (not shown) is inserted into the flush bolt hole 6, whereby the first member 1 and the second member 2 are fastened to each other.
The flush bolt hole 6 is formed by a drill or the like. In order to properly fasten the members, it is important that the shape of the flush bolt hole 6 is accurately formed with the desired shape. For this reason, after the formation of the flush bolt hole 6, the shape of the flush bolt hole 6 is measured. Specifically, the countersunk head depth and the hole diameter of the flush bolt hole 6 are measured. The countersunk head depth is a depth h of a portion in which a diameter becomes the reference diameter determined in advance in the countersunk head portion 7, and the hole diameter is a diameter in the constant portion 8.
FIG. 2A is a cross-sectional view showing a method of measuring the countersunk head depth. As shown in FIG. 2A, at the time of the measurement of the countersunk head depth, a step gauge 9 is used. The step gauge 9 is provided with a rod, and the countersunk head depth is measured by inserting a tip portion of the rod into the flush bolt hole 6 which is a measurement target. Further, FIG. 2B is a cross-sectional view showing a method of measuring the hole diameter. At the time of the measurement of the hole diameter, a plunger gauge 10 is used, and similar to the time of the measurement of the countersunk head depth, the hole diameter is measured by inserting the tip of a rod provided at the plunger gauge 10 into the flush bolt hole 6.
In connection with the above, a cylindrical hole shape measuring machine is disclosed in PTL 1 (Japanese Unexamined Patent Application Publication No. 4-138301). The cylindrical hole shape measuring machine uses a pair of contacts and a pair of differential transformers which converts the amount of displacement of each contact into an electric signal.